The current crisis is CO₂, but suppose we will create a clean and cheap power source, and soon the next problem will be cooling off all that energy we produce, which gets released as heat. Then there is all the methane — even if we all go vegan, with the current growth of population just the amount produced by all the farting babies will be too much. Earth doesn't scale, without lowering the fertility rate we are doomed sooner or later, no matter how green and sustainable we become.